The present invention relates to a liquid ejection apparatus and a liquid pressure control method of the liquid ejection apparatus.
The term “liquid ejection apparatus” as used herein is not limited to recording apparatus such as an ink-jet recording apparatus, a copier, and a facsimile machine that perform recording on a recording subject member such as a recording sheet by ejecting ink from a recording head toward the recording subject member, but includes apparatus that cause, instead of ink, a liquid for a particular purpose to stick to an ejection target member corresponding to the recording subject member by ejecting the liquid from a liquid ejection head corresponding to the recording head toward the ejection target member. Examples of the liquid ejection head are, in addition to the above-mentioned recording head, a colorant ejection head that is used for manufacture of color filters of a liquid crystal display or the like, an electrode material (conductive paste) ejection head that is used for formation of electrodes of an organic EL display, a field emission display (FED), or the like, a bioorganic material ejection head that is used for manufacture of a biochip, and a sample ejection head as a precision pipette that ejects a sample.
Conventionally, ink-jet recording apparatus have been used widely as liquid ejection apparatus that eject liquid toward a target. More specifically, those ink-jet recording apparatus include a carriage, a recording head that is mounted on the carriage, and an ink cartridge that stores ink as liquid. Printing is performed on a recording medium as a target in such a manner that ink is supplied from the ink cartridge to the recording head and the ink is jetted from nozzles that are formed in the recording head while the carriage is moved relatively to the recording medium.
Among those ink-jet recording apparatus are ones in which the ink cartridge is not mounted on the carriage (what is called an off-carriage type) for the purposes of reducing the load of the carriage or reducing the size or thickness of the apparatus. Usually, such an ink cartridge has an ink pack that accommodates ink and a case that houses the ink pack.
The ink pack is crushed by supplying air that is pressurized by a pressure pump into the space between the ink pack and the case, whereby ink is supplied from the ink pack to the recording head that is mounted on the carriage.
For example, it is known to use a diaphragm pump as such a pressure pump (refer to JP-A-2000-352379, for example). More specifically, the diaphragm pump disclosed in JP-A-2000-352379 includes a pump chamber having a diaphragm, a suction unidirectional valve, and a discharge unidirectional valve. The capacity of the pump chamber is varied by deformation of the diaphragm.
The suction unidirectional valve, which is disposed between the pump chamber and the outside (i.e., atmosphere) is opened and thereby allows air to flow into the pump chamber only when the pressure in the pump chamber has become approximately lower than atmospheric pressure. Further, the suction unidirectional valve is closed and thereby prevents the air from flowing out of the pump chamber to the outside when the pressure in the pump chamber has become approximately higher than atmospheric pressure. As such, the suction unidirectional valve is a valve for allowing passage of only air that is going to flow into the pump chamber from the outside and for stopping air that is going to flow in the opposite direction.
The discharge unidirectional valve, which is disposed between the pump chamber and the space between the ink pack and the case, is opened and thereby allow air to flow out of the pump chamber to the space between the ink pack and the case only when the pressure in the pump chamber has become higher than the pressure of the space between the ink pack and the case. Further, the discharge unidirectional valve is closed and thereby prevents air from flowing backward from the space between the ink pack and the case into the pump chamber when the pressure in the pump chamber has become lower than the pressure of the space between the ink pack and the case. As such, the discharge unidirectional valve is a valve for allowing passage of only air that is going to flow into the space between the ink pack and the case from the pump chamber and for stopping air that is going to flow in the opposite direction.
In the diaphragm pump having the above structure, as the capacity of the pump chamber is increased and decreased repeatedly, air is sucked through the suction unidirectional valve and the sucked air is pressurized and sent to the space between the ink pack and the case.
Incidentally, the diaphragm pump disclosed in JP-A-2000-352379 is configured in such a manner that the air in the space between the ink pack and the case cannot flow backward into the pump chamber. Therefore, the air in the space between the ink pack and the case is always in a pressurized state. This results in a problem that the pressurized air in the space between the ink pack and the case expands the case to possibly cause an event that the ink cartridge is hard to remove from a main body case of the inkjet recording apparatus when it is attempted to do so.
Further, the ink pack is always in a pressurized state, too. This results in a problem that ink may leak out of a connecting portion between the ink pack and an ink supply tube or the like when the ink cartridge is removed from the main body case of the ink-jet recording apparatus.
In view of the above, it has been conceived to provide, in a certain portion between the diaphragm pump and the ink cartridge, an air release valve or the like capable of making the air pressure in the ink cartridge equal to atmospheric pressure. This measure can solve the above problems because the air pressure in the ink cartridge can be made equal to atmospheric pressure when the ink cartridge is removed from the main body case of the ink-jet recording apparatus.
However, the provision of the air release valve increases the number of components of the entire apparatus and hence may lower the production efficiency or increase the space occupied by the apparatus. In a case that the air release valve is an electromagnetic valve or the like, the control may be complicated.
Liquid ejection apparatus as typified by ink-jet recording apparatus incorporate a liquid ejection head for ejecting liquid toward an ejection target member, and a large number of liquid ejection nozzles for ejecting very small amounts of liquid jets are arranged on a head surface that is opposed to the ejection target member. For example, the liquid ejection head is configured in such a manner that piezoelectric elements are provided in pressure chambers that are provided adjacent to the nozzle openings of the liquid ejection nozzles, respectively. When an electrical signal is applied to a piezoelectric element, the piezoelectric element expands or contracts, as a result of which the liquid pressure in the pressure chamber is varied and a very small amount of liquid is ejected from the nozzle opening. In general, a negative pressure that is caused by expansion or contraction of the pressure chamber that is provided adjacent to the nozzle opening of each liquid ejection nozzle allows liquid to be supplied to (i.e., sucked into) the liquid ejection nozzle from a liquid storage portion that liquid-communicates with the liquid ejection nozzle. However, in liquid ejection apparatus such as large-size ink-jet recording apparatus in which the recording head (liquid ejection head) and the ink cartridge (liquid storage portion) are distant from each other and liquid-communicate with each other via a hollow tube or the like, the negative pressure occurring in each liquid ejection nozzle at the time of liquid ejection may be insufficient to supply a necessary and sufficient amount of ink (liquid) from the ink cartridge to the liquid ejection nozzle through suction. In view of this, various measures have been taken in such ink-jet recording apparatus. For example, the ink cartridge is provided with a mechanical pressure applying member such as a leaf spring and ink is pressurized and sent from the ink cartridge to the recording head by spring force of the lead spring. In another example, ink is pressurized and sent by utilizing the siphon phenomenon of atmospheric pressure (refer to, for example, JP-A4-366643, which has a corresponding U.S. Pat. No. 5,453,770 and a corresponding European patent No. 518,380).
However, in the liquid ejection apparatus in which liquid is pressurized and sent from the liquid storage portion to the liquid ejection head by the mechanical pressure applying member such as a leaf spring or by utilizing the siphon phenomenon of atmospheric pressure, the liquid supply pressure cannot be controlled. This may result in an event that the pressure of the ink supply to the recording head has dispersion or the ink supply pressure varies to a large extent due to variation of the ink residual amount. It is therefore difficult to continue to supply ink from the ink cartridge to the recording head with the ink supply pressure kept stable. In previous recording apparatus such as ink-jet recording apparatus, it is rare that deterioration of the recording image quality due to such dispersion or variation of the ink supply pressure becomes a problem. However, in recent years, the image quality of ink-jet recording apparatus has been improved greatly, their recording execution speeds have been increased, and the liquid ejection accuracy that is required for various liquid ejection apparatus using the ink-jet technology has been increased. For these reasons, lowering of the liquid ejection accuracy due to dispersion or variation of the ink supply pressure is becoming a problem.